A conceptual study of the impacts of human activities on the natural ecology of estuaries was carried out. The Mersey Estuary was used as a case study. Environmental Impact Assessment Techniques (EIA) was used to identify and evaluate impacts generated by various activities of Man on estuaries. The study was conducted with materials and data gathered from scientific papers, documents, reports and other related literature sources. The findings of the research reveal that estuaries exist in several forms and shape. They are characterised by graded salinity ranging from marine condition (3.5%) to fresh water (0.05%) and periodic and spontaneous tidal movements. Estuaries are highly productive ecosystems and support large wildlife and fish. For many years Man has exploited estuarine resources. However, in the last two hundred years, the advent of the industrial revolution led to dramatic growth of navigation and establishment of industries close to estuarine waters. Consequently estuaries attracted large human populations which discharged sewage along with industrial effluent direct into their waters causing rapid deterioration in water quality and severe distress of the biota. The Mersey Estuary is typical of such industrialized estuaries. The level of engineering modification and the extent of pollution witnessed in the Estuary is possibly unparalleled by any other estuary in the U. K. These activities have significantly affected the ecosystem of the Mersey and the socioeconomic lif e of people living within its catchment boundary and beyond. EIA has become an important tool in environmental planning and management. I have used its principles and techniques in the identification and analysis of impacts caused by activities of Man on the Mersey Estuary. Analysis of results indicate that the construction of training wall along the sea channels and the building of the Manchester Ship Canal were most important, reducing estuarine capacity and the stabilization of the inner estuary navigation channel. The heavy organic load from sewage and industry discharged directly into the estuary caused severe deficiency of dissolved oxygen and consequent loss of fish and wildlife. The drive to clean up the Estuary started in the 1970s and so far positive changes are being recorded showing improved wildlife which reflect general improvement in the whole Estuary.